Process of making thin film high efficiency solar cells

ABSTRACT

Process of making thin film materials for high efficiency solar cells on low-cost silicon substrates. The process comprises forming a low-cost silicon substrate, forming a graded transition region on the substrate and epitaxially growing a thin gallium arsenide film on the graded transition region. The process further includes doping the thin gallium arsenide film and forming a junction therein. The graded transition region preferably is a zone refined mixture of silicon and germanium characterized by a higher percentage of germanium at the surface of the region than adjacent the substrate. The process also includes the formation of homojunctions in thin gallium arsenide films. 
     Solar cells made from the materials manufactured according to the process are characterized by a high conversion efficiency, improved stability and relatively low unit cost.

This is a division, of application Ser. No. 209,541 filed on Nov. 20,1980, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a process of making thin filmmaterials for solar cells and, more particularly, to a process of makingthin film gallium arsenide materials for high efficiency solar cells onlow-cost silicon substrates.

2. The Prior Art

For the large scale photovoltaic conversion of sunlight into electricpower, very low-cost yet high efficiency solar cells are needed. Today,most solar cells are made from single-crystal silicon material. Thesesingle-crystal silicon materials are anything but low-cost, however.Furthermore, the maximum theoretical conversion efficiency forsingle-crystal silicon solar cells under maximum illumination with solarlight on the ground at sea level (AM1) is about 23%. Solar cells madefrom single-crystal gallium arsenide (GaAs) materials, on the otherhand, have a maximum theoretical conversion efficiency under maximumillumination on AM1 of about 27%, i.e., higher than that forsingle-crystal silicon cells. Because of this high efficiency and due toits physical properties, gallium arsenide comes close to the optimum forsolar cell materials. Owing to the high absorption coefficient forvisible light of a GaAs cell, all light is absorbed in a surface layerof the cell not more than about one millimicron (μm) thick. Furthermore,at high temperatures, particularly above 100° C., the performance ofGaAs solar cells is better than that of silicon cells. The reason forthis better performance is twofold. First, the voltage decrease withtemperature is about 2.6 mV per °C. of temperature increase for a GaAscell, a voltage decrease which is less than that for a silicon cell. Asa result, the power of GaAs cells decreases less per °C. temperatureincrease than that of silicon cells. Second, the open circuit voltage ofGaAs cells at room temperature is only slightly less than one volt,which is appreciably higher than that of silicon cells. Consequently,the voltage decrease as a percentage of the original voltage per °C. oftemperature increase is comparatively low for GaAs cells.

Material consumption per unit of GaAs solar cells is high, however,because a monocrystalline GaAs substrate is needed. In addition, highpurity arsenic is a rare and expensive element while high purity galliumis not so rare although also very expensive. Gallium arsenide materialsfirst must be transformed into the single-crystal state before highefficiency solar cells can be made therefrom. Polycrystalline GaAs filmsnot only result in low efficiency cells but also exhibit instability.Attempts to grow thin film GaAs materials on foreign substrates so as toreduce material consumption per unit cells have been frustrated thus farprimarily due to grain size limitations and substrate-filmcontamination. Presently known fabrication techniques, therefore, holdlittle promise for the economic large-scale production of GaAs materialsfor use as high efficiency, low-cost solar cells.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to overcome the aboveshortcomings by providing a process for the large-scale manufacture oflow-cost thin film materials for high efficiency solar cells.

More specifically, it is an object of the present invention to provide aprocess of making low-cost, thin film, single-crystal gallium arsenidematerials for high efficiency solar cells on low-cost siliconsubstrates. The process essentially comprises forming a low-cost siliconsubstrate, forming a graded transition region on the substrate andepitaxially growing a thin gallium arsenide film on the gradedtransition region. Preferably, the silicon substrate is either alow-cost single-crystal silicon or a metallurgical grade polycrystallinesilicon. Preferably, the graded transition region is a zone refinedmixture of silicon and germanium characterized by a higher percentage ofgermanium at the surface of the region that adjacent the substrate.Preferably the epitaxial growing of the thin gallium arsenide film onthe transition region includes depositing the thin gallium arsenide filmpreferably in the amorphous state and transforming this amorphous filmto a single-crystal gallium arsenide structure by a directed energyprocessing technique. Preferably, such a directed energy processingtechnique involves the application of a large-area pulsed electron beamto the amorphous gallium arsenide film. Upon exposure to the pulsedelectron beam, the GaAs film melts and recrystallizes in asingle-crystal structure.

Solar cells made from the thin film materials manufactured according tothe process of the invention are characterized by a high conversionefficiency, improved stability and relatively low unit cost.

Other objects of the present invention will in part be obvious and willin part appear hereinafter.

The invention accordingly comprises the process and the product of thepresent disclosure, its components, parts and their interrelationships,the scope of which will be indicated in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the presentinvention, reference is to be made to the following detaileddescription, which is to be taken in connection with the accompanyingdrawings, wherein:

FIG. 1 is a schematic cross-sectional view of a representative highefficiency, thin film, single-crystal solar cell structure made fromthin film materials manufactured according to the process of theinvention;

FIG. 2 is a schematic cross-sectional view of a second representativehigh efficiency, thin film, single-crystal solar cell structure madefrom thin film materials manufactured according to the process of theinvention;

FIG. 3 is a schematic cross-sectional view of a third representativehigh efficiency, thin film, homojunction, single-crystal solar cellstructure made from thin film materials manufactured according to theprocess of the invention;

FIG. 4 is a germanium-silicon phase diagram; and

FIG. 5 depicts ion implantation profiles of germanium directly intosilicon and of germanium into 300 Angstroms (A) of germanium depositedon silicon.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In general, the present invention provides a process for the large-scalemanufacture of low-cost thin film materials for high efficiency solarcells. Preferably, the thin film materials are made of thin galliumarsenide (GaAs) films epitaxially grown on low-cost silicon substrateswith the aid of a graded transition region. The graded transition regionpreferably is a zone refined mixture of silicon (Si) and germanium (Ge)characterized by a higher percentage of germanium at the surface of theregion that adjacent the silicon substrate. The process also includesthe formation of homojunctions in gallium arsenide films.

A salient feature of the process of the invention is the use of directedenergy processing techniques. The preferred directed energy processingtechnique involves the use of large-area pulsed electron beams. Pulsedelectron beam processing is inherently a low-temperature process,reducing thereby the likelihood of contamination between heterostructurelayers. The employment of large-area (three inches or better) pulsedelectron beams to form single-crystal heterostructures withhomojunctions allows the low-cost, large-scale manufacture of thin filmgallium arsenide solar cells characterized by high conversion efficiencyand improved cell stability. Thin film gallium arsenide solar cells madefrom the thin film materials manufactured according to the process ofthe invention generally exhibit better than eighteen percent (18%)conversion efficiencies under maximum illumination with solar light onthe ground at sea level (AM1).

The use of pulsed electron beam processing not only allows the epitaxialgrowth of thin films at low temperatures but it also permitsion-implanted junctions and homojunctions to be annealed at lowtemperatures. (The terms "epitaxy" and "epitaxial growth" as used hereinare intended to define the growth of one crystal on the surface ofanother crystal in which the growth of the deposited crystal is orientedby the lattice structure of the substrate. The term "epitaxial layer" asused herein is intended to define a semiconductor layer having the samecrystalline orientation as the substrate on which it is grown.)Processing the thin films at low temperatures, in turn, allows the useof much less demanding film deposition techniques. This is so sincepulsed electron beam (PEB) epitaxy results in crystalline structures tobe formed in the thin films independent of the film deposition techniqueand independent of the treatment of the substrate. For instance,low-temperature chemical vapor deposition (CVD) resulting inpolycrystalline films, evaporation, sputtering, plasma ion depositionand other deposition techniques can all be considered in the process ofthe invention. All these deposition techniques can be used because, uponexposure to a pulsed electron beam, the deposited thin films melt andthen regrow in liquid-phase epitaxy (LPE) to single-crystal structure.In addition, low temperature processing has the advantages of reducedstress at interfacial layers, reduced autodoping of one layer by anadjacent layer, and improved crystallinity. Ion implantation of the thinfilm materials, followed by electron beam annealing, also makes forregions of doping concentrations above the solid solubility level thatis achievable with conventional furnace annealing techniques.

It is the employment of the graded transition region on top of thesilicon substrate, however, that renders the silicon substrate receptivefor the epitaxial growing of the thin gallium arsenide film thereon. Forit is this graded transition region that provides for lattice constantmatching between the silicon substrate and the thin gallium arsenidefilm. And without this lattice constant matching, the pulsed epitaxialgrowing of the thin gallium arsenide film on the silicon substrate wouldnot be feasible. (The term "lattice constant" as used herein is intendedto define a parameter defining the unit cell of a crystal lattice, i.e.,the length of one of the edges of the cell or an angle between the edgesof the cell.) The lattice constant for silicon is 5.431 Angstroms andfor gallium arsenide it is 5.653 Angstroms. This represents aconsiderable mismatch between the silicon and the gallium arsenide. Thelattice constant for germanium is, on the other hand, 5.657 Angstroms.Thus, only 0.004 Angstroms separate the lattice constant of galliumarsenide from that of germanium--a nearly perfect lattice constant matchbetween GaAs and Ge. As mentioned, the graded transition regionpreferably is a zone-refined mixture of Si and Ge characterized by ahigher percentage of germanium at the surface of the transition regionthan adjacent the silicon substrate.

The combined effect of having a graded transition region and employinglarge-area pulsed electron beam processing in the process of theinvention permits the use of inexpensive silicon substrates. The siliconsubstrate need only be of large-area crystalline structure and besufficiently conductive to perform as the back surface contact for thethin GaAs film solar cell made from the thin film materials manufacturedaccording to the process of the invention. The purity or electricalquality of the silicon substrate, therefore, is of little, if any,consequence since impurities from the silicon substrate will not diffuseinto the thin GaAs film during the processing. The use of inexpensivesilicon substrates, such as metallurgical grade polycrystalline silicon,in the process of the invention significantly reduces, however, thematerial consumption of thin GaAs films per unit of GaAs solar cells.

The process of making thin film GaAs materials for high efficiency solarcells according to the invention is best described in detail withreference to the drawings. In FIGS. 1-3 are shown schematiccross-sectional views of representative high efficiency solar cells madefrom thin film materials manufactured according to the process of theinvention.

A representative high efficiency solar cell structure 10 is depicted inFIG. 1. Solar cell structure 10 comprises an inexpensive siliconsubstrate 12, a graded transition region 14, a thin GaAs film 16 havinga p-region 18 and an n-region 20 separated by a p-n barrier junction 22,a conductive anti-reflective (AR) coating 24, and a front ohmic contactgrid 26.

FORMATION OF THE TRANSITION REGION

As stated, the silicon substrate 12 is formed of a low-cost, large-areacrystalline structure, such as a single-crystal Czochralski-grownsilicon material. The substrate 12 is formed with little regard for itspurity or electrical quality except insofar that it be sufficientlyconductive to function as the back surface ohmic contact for the solarcell 10. The graded transition region 14 is formed on top of the siliconsubstrate 12. As stated, the graded transition region 14 is a zonerefined mixture of silicon and germanium characterized by a higherpercentage of germanium at the top surface of region 14 than adjacentthe silicon substrate 12. The function of this transition region 14 is,as also stated, to achieve lattice constant matching with the thin GaAsfilm 16. The transition region 14 is formed by the application of pulsedelectron beam epitaxy of germanium on silicon. First, a thin, amorphouslayer of germanium is deposited on top of the silicon substrate 12.Deposition of the amorphous germanium layer can be effected in aplurality of ways. One preferred way is by evaporation of the amorphousgermanium layer in a suitable vacuum environment. The deposited Ge layeris then subjected to submicrosecond pulsing by a large-area, highintensity electron beam. The pulse of thermal energy from the electronbeam causes the thin amorphous Ge layer and a thin silicon layer at theGe-Si interface to melt. The melt front is caused to propagate normallyin the plane of the Ge-Si interface from the underlying silicon layer tothe overlying germanium layer. When this melt region resolidifies, itresolidifies epitaxially. Not only is there a thorough Ge-Si mixing atthe interface, but in addition pulse recrystallization is achieved and ahigh-quality heteroepitaxial layer is formed. In this firstrecrystallized heteroepitaxial layer, there is proportionally moresilicon present than in the subsequently formed layers making up thetransition region 14. The Si-Ge transition region 14 preferably isformed in a multistep process. That is, after the above deposition andpulsing of the first thin, amorphous Ge layer, the deposition andpulsing steps are repeated until a graded, zone refined mixture of ahigh-quality Si-Ge transition region 14 has been achieved. With eachsubsequent deposition of a thin, amorphous Ge layer and its pulsedrecrystallization, progressively more and more germanium will be presentin the transition region 14 until finally at the top surface of theregion 14, the top recrystallized heteroepitaxial layer will containmostly germanium and very little silicon. This is a result of the zonerefining of the germanium and is best described with reference to FIG.4.

FIG. 4 depicts the phase diagram of a mixture of germanium and siliconupon heating and cooling. The phase diagram plots the relative atomicpercentages of germanium and silicon in the mixture against temperaturein the range of 900° C. to 1500° C. As may be observed in this phasediagram, when germanium and silicon are thoroughly mixed and meltingoccurs, a motion of the liquid-solid (i.e., freezing) interface uponcooling causes zone refining of the germanium from the substrate 12toward the top surface of the transition region 14. The result isgermanium enrichment at the top surface of the transition region 14.Since, as already stated, germanium represents a near-perfect latticeconstant match to gallium arsenide, the transition region 14 renders thesilicon substrate 12 receptive for the epitaxial growing thereon of thegallium arsenide films, as more fully described below.

A variation in the deposition by evaporation of the amorphous germaniumlayers on the silicon substrate 12 is represented by "stitching" thelayers in place. This "stitching" of the amorphous Ge layers at selectedpoints to the substrate 12 is effected by impinging high energy ions atthe Ge layers at those selected points. This "stitching" approach withhigh energy ions already causes a mixing of the germanium and thesilicon at their interfaces even before the pulse melting thereat by theapplication of PEB epitaxy. This mixing effect can best be observed inFIG. 5. FIG. 5 shows the ion implantation profiles (the shaded area) of50 keV Ge, 1×10¹⁵ /cm², first into silicon directly, and second into a300 Angstrom thick layer of Ge deposited on Si. FIG. 5 plots the depth(in μm) of ion penetration by the Ge against the concentration (in 10²¹/cm³).

Another preferred way of depositing the amorphous germanium layer on topof the silicon substrate 12 is by low-temperature chemical vapordeposition (CVD). Such low-temperature CVD of the amorphous germaniuminitially results in a polycrystalline layer of Ge. Upon pulsing withthe large-area, high intensity electron beam, however, the Ge layer isnot only zone refined with the silicon substrate 12 but is alsorecrystallized epitaxially into a high-quality, single-crystal Si/Getransition region 14 on the silicon substrate 12.

FORMATION OF THE THIN GaAS FILM

With the formation of the graded transition region 14 characterized byan enriched germanium layer at the top surface of the region 14, thesilicon substrate 12 has been rendered receptive to the formationthereon of the thin GaAs film 16. The preferred approach to theformation of this GaAs film 16 is quite similar to the formation of theSi/Ge transition region 14 above discussed. One or more thin films ofGaAs are first deposited on top of the transition region 14. Thedeposited thin GaAs films are then subjected to pulsed electron beam(PEB) processing in order to regrow epitaxially the GaAs films into ahigh-quality, single-crystal structure. Again, a number of filmdeposition techniques can be used to deposit the GaAs films because thePEB epitaxy process is insensitive to film structure or contamination.Either evaporation or low-temperature CVD is used to deposit the thinGaAs films on the Ge rich surface of the transition region 14. If forsome reason adequate pulsed epitaxial growth of the thin GaAs film uponthe Ge rich surface of the transition region 14 is not achieved,metal-organic chemical vapor deposition (MO-CVD) is then preferablyused. MO-CVD effects the deposition of the thin GaAs film at highertempeature than CVD and thus achieves direct epitaxial grownt of theGaAs film on the transition region 14. The advantages of pulsed electronbeam epitaxial growth processing include: reduced thermal stresses thatmight otherwise be generated at the interface of the GaAs film 16 and ofthe Ge rich surface of the transition region 14 and, more importantly,reduced diffusion of Ge into the GaAs film 16. This advantageous dualresult flows from the fact that PEB epitaxy is essentially a lowtemperature process and whose operating parameters are so chosen thatthe Ge rich surface of the transition region 14 remains essentially coldduring the processing. The result is a high-quality, single-crystal,thin film GaAs structure free of contamination and of thermal stresseffects.

The thin GaAs films 16 preferably are doped with one dopant ion speciesduring the deposition step, as is done conventionally. Other preferredmethods of doping of the thin GaAs films 16 include ion implantationfollowed by pulsed electron beam annealing and PEB diffusion ofsurface-deposited thin doping film. Each of these two latter methodsexcels in achieving precise, highly concentrated doping of the thin GaAsfilms 16. Due to such high-level doping, concentrations of dopantspecies higher than solid solubility levels are achieved in the thinGaAs films 16. These high concentrations of dopant species provide, inturn, very high conductivity in the doped regions of the solar cell 10structure.

Formation of the p-n junction barrier 22 in the doped thin GaAs film 16is also preferably effected either by ion implantation followed by PEBannealing or by PEB diffusion of surface-deposited thin doping film. Thedeposition of the doping films preferably is effected by evaporation,followed by pulsing with an electron beam to effect the diffusion of thedopant ion species from the doping films into the thin GaAs film 16.

The first surface ohmic contact grid 26 preferably is formed in eitherof two ways. The first is by ion implantation for very high dopantconcentrations to effect ohmic contact with evaporated metallization.The second is by pulsed electron beam sintering of metallizationpatterns evaporated on the GaAs film 16. The solar cell 10 structure issurface finished by the application thereon, as by evaporation orsputtering, of the conductive anti-reflective (AR) coating 24.

A second representative high efficiency solar cell 30 is shown in aschematic cross-sectional view in FIG. 2. Solar cell 30 is in allessential respects similar in construction to the solar cell 10 shown inFIG. 1. Solar cell 30 comprises a low-cost silicon substrate 32, agraded transition region 34, a thin GaAs film 36 having an n-region 38and a p-region 40 separated by a p-n barrier junction 42, a frontsurface ohmic contact grid 44, and a gallium-aluminum-arsenide(GaAlAs)window 46 in lieu of the conductive AR coating 24. Preferably, thelow-cost silicon substrate is a metallurgical grade or upgradedmetallurgical grade silicon, such as for example, Wacker castpolycrystalline silicon. The graded transition region 34 is identical tothe graded transition region 14 of the solar cell 10 and is formed inthe same way. Like transition region 14, region 34 is a zone refinedmixture of silicon and germanium characterized by a higher percentage ofGe at the surface of the region 34 than adjacent the substrate 32. Thus,the region 34 renders the underlying low-cost Si substrate 32 receptivefor the epitaxial growth thereon of the thin GaAs film 36 in the sameway as the GaAs film 16 was grown in the solar cell 10. Only the dopingdifferentiates film 36 from film 16 in that the respective p and nregions thereof are reversed in position. Also, the front surface ohmiccontacts 44 are formed in the same way as the contacts 26 of the cell10. However, in place of the conductive AR coating, a GaAlAs window 46is formed, again by PEB diffusion in a low temperature process. TheGaAlAs window 46 improves the surface recombination velocity, hence theconversion efficiency of the resultant solar cell 30. Window 46preferably is formed by depositing, such as by evaporation, thin filmsof aluminum over the thin GaAs film 36 and then diffuse and therebyalloy the aluminum into the GaAs film 36 using a large-area,high-intensity pulse of electrons. This low-temperature approach isadvantageous in that it precludes autodoping and film contamination. TheGaAlAs window 46 can also be formed by high-current ion implantation,followed by electron beam pulsing.

A third representative, thin film, single-crystal high-efficiency solarcell 50 is shown in FIG. 3. Solar cell 50 comprises a low-cost, such asmetallurgical grade, silicon substrate 52, a graded Si-Ge transitionregion 54, a thin GaAs film 56 having a p⁺ region 58, a P region 60 andan n⁺ homojunction 62. A front surface contact grid 64 and a GaAlAswindow 66 formed on the thin GaAs film 56 complete the solar cell 50.Thus, the structure of the thin GaAs film 56, particularly the additionof the homojunction 62, is what differentiates the representative solarcell 50 from the cells 10 and 30 above described. The addition of thehomojunction 62 further contributes to increase the already highconversion efficiency of the resultant solar cell 50. The homojunction62 is also responsible for improving the stability of the cell 50.During the initial formation of the thin GaAs film 56, by the epitaxialgrowth thereof on the germanium-rich surface of the graded transitionregion 54, a back surface field preferably is formed in the p⁺ region58. This back surface field is formed by the diffusion of germanium intothis p⁺ region 58 during its pulsing with the high-intensity electronbeam.

FORMATION OF THE HOMOJUNCTION

Preferably, the homojunction 62 is formed in the third representativehigh efficiency solar cell 50 as follows. Following the preferredautodoping in the formation of the p⁺ region 58, a lightly doped Pregion 60 of GaAs film is formed on the region 58 by FEB epitaxy. Then,a heavily doped n⁺ homojunction 62 is produced preferably by ionimplantation using selenium (Se) ions. Selenium ions are preferredbecause of the close lattice match of a selenium ion substituting intoan arsenide lattice site in gallium arsenide. Choice of a matched ionresults in minimum lattice strain in the thin GaAs film 56, avoidingthereby undesirably high recombination rates. The ion-implanted seleniumin the GaAs film region 62 is pulse electron beam annealed for ionactivation and carrier concentration. The implanted and annealed region62 preferably is then etched and oxidized to produce a shallowhomojunction with a thin anodic oxide AR coating. The homojunction layer62 alternatively is formed by depositing a thin selenium film on thethin GaAs film 56. The deposited selenium film is then irradiated by apulsed electron beam to cause the selenium ions to diffuse, that is, tomigrate into the gallium arsenide film 56. Deposition of the seleniumfilm is preferably effected by evaporation or by low temperature CVD.

The front surface ohmic contacts 64 and the GaAlAs window 66 are formedon top of the homojunction layer 62 of the thin GaAs film 56 in the samemanner as the contacts 44 and the window 46 were formed in the solarcell 30 of FIG. 2.

CONCLUSION

Thus it has been shown and described a cost-effective process for thelarge-scale manufacture of thin film gallium arsenide materials usefulfor the making of high efficiency single-crystal solar cells on low-costsilicon substrates, which process satisfies the objects and advantagesset forth above.

Since certain changes may be made in the present disclosure withoutdeparting from the scope of the present invention, it is intended thatall matter described in the foregoing specification or shown in theaccompanying drawings, be interpreted in an illustrative and not in alimiting sense.

What is claimed is:
 1. A process of forming a high efficiencysingle-crystal, thin film, homojunction gallium arsenide solar cell on asingle-crystal silicon substrate comprising:(a) forming a single-crystalsilicon substrate; (b) forming a graded transition region on saidsubstrate by deposition of a thin layer of germanium directly on saidsilicon substrate and pulsing said layer with an electron beam toeffectuate thereby zone refining and mixing of said germanium layer andsaid silicon substrate; (c) epitaxially growing a thin gallium arsenidefilm on said transition region; (d) doping said thin gallium arsenidefilm with one dopant species; (e) forming a homojunction in said thin,doped gallium arsenide film by introducing therein a second dopantspecies; and (f) forming front ohmic contacts on said gallium arsenidefilm.
 2. The process of claim 1 wherein said deposited thin layer ofgermanium is in the amorphous state.
 3. The process of claim 2 whereinsaid depositing said thin layer of germanium is effected bylow-temperature chemical vapor deposition.
 4. The process of claim 1wherein said depositing said thin layer of germanium is effected byevaporation.
 5. The process of claim 4 further including stitching theevaporated layer of germanium at selected points to said substrate byimpinging high energy ions at said layer at said selected points.
 6. Theprocess of claim 1 wherein said pulsing said layer of germanium causes amelting of said layer and of said silicon substrate at their interface,which molten interface upon cooling effects zone refining and mixing thegermanium with the silicon in said transition region from said substratetoward the surface of said transition region.
 7. The process of claim 6wherein said zone refining and mixing of the germanium with the siliconresults in a higher percentage of germanium at said transition regionsurface than adjacent said silicon substrate.
 8. The process of claim 1wherein said epitaxially growing and doping said thin gallium arsenidefilm, said forming said homojunction and said front ohmic contactsincludes low-temperature pulsed electron beam processing.
 9. A processof forming a high efficiency single-crystal, thin film, homojunctiongallium arsenide solar cell on a metallurgical grade silicon substratecomprising:(a) forming a metallurgical grade silicon substrate; (b)forming a graded transition region on said substrate by successivedepositions of thin layers of germanium, with each of said layers beingsuccessively pulsed with an electron beam, causing thereby a melting ofsubstantially equal thickness of said germanium layer and of saidsilicon substrate at their interface, which molten interface uponcooling effects zone refining and mixing of the germanium with thesilicon in said transition region from said silicon substrate toward thesurface of said transition region, said transition region comprising amixture having a higher percentage of germanium at the surface of saidtransition region than adjacent said silicon substrate; (c) epitaxiallygrowing a thin gallium arsenide film on said transition region; (d)doping said thin gallium arsenide film with one dopant species; (e)forming a homojunction in said gallium arsenide film by introducingtherein a second dopant species; and (f) forming front contacts and agallium-aluminum-arsenide window on said gallium arsenide film.